Yes that sounds like the old original primer pump. White knob, unscrews then pumps. They absolutely leak and need replacement. The only MB I bought with the original primer pump I replaced myself. Not hard to do at all. I think Mercedes Source has a video on it.

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Spencer

87 2.5L 190D Turbo 5-speed - 102k (Daily Driver)
82 300D Turbo - 145k - Light Ivory (near mint)

Gone: 85 300D Federal - mileage unknown